The present invention relates generally to a motorgrader having a two-part articulated frame defined by a rear drive unit and a front steering unit which can be rotated or pivoted relative to the drive unit and, more particularly, to an improved method and apparatus for controlling the cross slope angle cut by such a motorgrader while the motorgrader is making a turn and/or is traveling in a steep slope condition.
It is important to be able to grade surfaces during the construction of roadbeds, runways, parking lots and the like so that the grade and cross slope, i.e., the slope normal to the direction of travel of the motorgrader's blade, closely approximate the finished surface. In this way, the pavement is of a uniform thickness and strength. Highly skilled motorgrader operators can perform grading operations manually to produce acceptable grades and cross slopes. However, due to time pressures and the inherent risk of error in manually producing grades and cross slopes, automatic control systems have been developed to assist operators and reduce the time and skill required to obtain acceptable grading.
One system, which is disclosed in U.S. Pat. No. 4,926,948 and is owned by the assignee of the present invention, permits a motorgrader operator to preset the slope of the blade and maintain that slope even when the motorgrader is being operated in a "crabbed" steering position. While this system is a substantial improvement over previously available slope preset systems, it is not capable of maintaining the accuracy of the cut when the motorgrader is making a turn. The reason for this relates back to the fact that cross slope is defined as the slope normal to the direction of travel of the blade. When the motorgrader is making a turn, the blade normally has a direction of travel that differs from the direction of travel of the remaining portions of the motorgrader. This control system, however, does not take into consideration the blade's specific direction of travel when the motorgrader is turning. Thus, this system is not able to accurately control the cut of the blade when the motorgrader is making a turn.
While motorgraders normally travel in a relatively horizontal plane during operation, there are situations when a motorgrader is operated in a steep slope condition. For example, a motorgrader might be operated on a side slope or could be driven up or down a steep hill. The system disclosed in U.S. Pat. No. 4,926,948 does not totally correct for errors occurring when the motorgrader is not traveling in a horizontal plane. This is because cross slope is referenced to gravity, yet the rotational angle sensors disclosed in U.S. Pat. No. 4,926,948 are referenced to the mainframe of the motorgrader, which does not always operate in a horizontal plane. While such errors are normally insignificant, they can become problematic if the motorgrader is operated on a steep hill or side slope.
Accordingly, there is a need for an improved method and apparatus for operating a motorgrader having a two-part articulated frame to maintain a desired cross slope when the motorgrader is making a turn and when the motorgrader is operated in a steep slope condition.